1. Technical Field
The present invention relates to a differential apparatus used for a vehicle or the like and more particularly, it relates to a mechanism for limiting differential operations of the differential apparatus.
2. Description of the Related Art
Such a differential apparatus provided with a differential limiting mechanism can be found in the publication, Japanese Patent Publication (Kokai) No. 63-76938. The differential apparatus includes a differential casing driven by an engine through a drive gear and a ring gear. About an axis of the differential casing, two opposing (left and right) helical side gears are arranged coaxially with the axis. In the differential casing, two pairs of bores are formed outside of the side gears to run parallel with the axis of the differential casing. In these bores, helical pinion gears with six teeth are rotatably accommodated, respectively. Each of the helical pinion gears consists of an axial outside portion engaging with each side gear and an axial inside portion engaging with an axial inside portion of the other pinion gear. Consequently, the side gears are associated with each other through the helical pinion gears. Note, in the differential apparatus, a spacer is disposed between the side gears, provided on a periphery thereof with arc recesses opposing the bores, respectively.
The above-mentioned differential limiting mechanism of the apparatus operates as follows.
When there is produced a difference in driving resistance between the right and left side gears, driving force transmitted from the engine is divided into the side gears differently by the rotations of the pinion gears. Then, each pinion gear is forced against the cylindrical inner wall of the bore and the arc recess of the spacer by the meshing reaction of the pinion gear with the other pinion gear, so that a frictional resistance is produced. Furthermore, by thrust forces caused by the meshing operations of these helical gears, i.e., the side gears and the pinion gears, another frictional resistance is generated between the respective helical gears and the differential casing including the spacer. In this way, a differential limiting force can be obtained by these frictional resistances.
In connection with the above-mentioned differential limiting mechanism, a method of increasing the differential limiting force is also known. In this method, owing to the meshing reaction of the side gears with the pinion gears, which is caused by keeping a portion of each side gear rotatably supported by the differential casing in a free-fitting condition, the tip surfaces of the side gear are urged against the cylindrical wall of the differential casing thereby to produce the frictional resistance.
In the differential apparatus mentioned above, however, the helical side gears and the helical pinion gears are selected in accordance with a "gear-standard" defining basic gear specifications. For example, it is noted that, on general conditions of six in number of teeth, 20.degree. in pressure angle and 2.25 module in tooth length, a tip circular thickness of the pinion gears is established to be around 20 percent of a tooth length thereof.
Therefore, if the pinion gear having such a tip circular thickness is slid and rotated in the bore formed in the differential casing in order to obtain the differential limiting force, there would be raised a problem that surface pressure of the pinion gear is excessive to cause a seizure phenomenon because of an insufficiency of the tip circular thickness.
In order to reduce the surface pressure of the pinion gear, it may be thought that a diameter of each bore of the differential casing is increased while a number of teeth of the pinion gear is increased to be seven or eight, which would be around 22 percent and 24 percent in tip circular thickness, respectively, on the same conditions as above. In either of the cases, however, there is still remained a problem of "seizure phenomenon" between the tip surfaces of the pinion gears and the bores, so that the apparatus would be large-sized.
Also in the side gears of the differential apparatus where the differential limiting force is increased by frictionally rotating the side gears in the differential casing, there is still remained a problem of increasing the surface pressure of the side gears thereby causing the seizure.